BER estimation for MIMO HSDPA : Chip level Wiener equalizer and successive interference cancellation

Abstract

In this paper we describe and simulate an accurate Bit Error Rate (BER) estimator for a chip level Wiener equalizer and combined equalizer/interference canceller for employment with the CDMA MIMO High Speed Downlink Packet Access (HSDPA) feature of the Third Generation Partnership Project (3GPP). In the 2 × 2 MIMO system two data streams may be transmitted in the downlink to each user equipment (UE) using assigned multiple orthogonal CDMA codes of Spreading Factor 16. The two streams are mapped to the two transmit antennas via a 2 × 2 weight matrix. Since the two streams use the same set of CDMA codes, it results in some direct interference between the two data streams at the receiver. The signal constellations from the two data streams interfere with each other and this complicates the BER estimation process. Treating this interference as AWGN is not sufficient for accurate BER estimation. The modulation type and order needs to be taken into account. Furthermore, the modulation order of the two data streams may also be different, e.g., QPSK may be used for one stream while 16QAM is used for the other. In this paper we describe the HSDPA transmitter and antenna weight matrix parameter calculations, as well as a chip level equalizer structure for which the BER is to be estimated. The transmit antenna weight matrix calculations are based on Channel State Information (CSI) and can be used, along with adaptive modulation and coding to optimize transmission. In the receiver a chip level 2 × 2 matrix Wiener equalizer is first used to recover the two transmit antenna streams in a minimum mean square error sense. The equalizer is implemented using a generalized overlap-save filtering method using FFT processing. Following this, the transmitter processing is reversed to estimate the transmitted data. The BER estimator must take into account white noise at the receiver, the interference between codes from loss of orthogonality due to the multipath channel, self noise from different time shifts of the given user's code (also due to the multipath channel) and the crosstalk between the user's two data streams, which use the same CDMA codes. The BER estimator for both the basic Wiener filter receiver and the Wiener filter plus interference canceller will be analyzed and compared with simulated BER measurements.